1. Field of the Invention
The present invention relates to a method for manufacturing a microlens.
A microlens is extensively used in optic devices, such as an optical communication connector and a charge coupled device (CCD) image sensor. The polishing method or the mold method is used to manufacture a normal lens, but other methods, such as the reflow method or the grayscale mask method is generally used to manufacture a microlens. This is because a microlens is minute, compared to a normal lens. The following is a brief explanation of the reflow method. First, a nearly cylindrical resist pattern is formed on a base substrate with photolithography. Then, the resist pattern is fluidized by heating the base substrate, and a nearly hemispheric resist pattern is formed by the surface tension of the fluidized resist pattern. Next, the nearly hemispheric resist pattern and the base substrate are simultaneously etched with anisotropic dry etching. Then a nearly hemispheric pattern is printed on the base substrate, and thus a microlens is formed.
2. Background Information
Japanese Patent Publication JP-A-05-136460 (especially page 4 and FIG. 2), and Japanese Patent Publication JP-A-06-104480 (especially pages 3-5 and FIG. 4), each show a method for manufacturing a microlens.
As the second embodiment of a method for manufacturing a microlens, the Japanese Patent Publication JP-A-05-136460 describes a method for manufacturing a microlens by using a nearly hemispheric resist pattern, which is formed on a base substrate (GaAs) with the potting method, as an etching mask, and by etching the resist pattern and the base substrate simultaneously. Also, the Japanese Patent Publication JP-A-06-104480 describes a method for manufacturing a microlens basically with the hitherto known reflow method. However, a structural feature of the microlens is that the surrounding portion of the microlens is formed so that its height is higher than the top of the microlens. Because of this structure, the microlens is protected from damage caused by external factors. The following is a brief explanation of a method for manufacturing a microlens described in this Japanese Patent Publication. First, a base of a negative resist is formed on the surrounding part of the side of a base substrate (InP) where a microlens is formed. Then, a positive resist is formed on both the base of the negative resist and the central part of the base substrate where a microlens is formed, and the bowl-shaped positive resist is formed by fluidizing the positive resist with a thermal treatment. Next, a microlens is formed by etching this resist pattern and the base substrate simultaneously with anisotropic dry etching. At this time, if the etch selectivity between the positive resist and the base substrate nearly equals one and the etch selectivity between the negative resist and the base substrate nearly equals to three, the height of the surrounding portion of the microlens is formed higher than the top of the microlens.
In the reflow method, the nearly cylindrical resist pattern formed by photolithography is fluidized with a high temperature heat treatment (i.e., a reflow treatment), and it is transformed into a nearly hemispheric resist pattern. In this reflow treatment, some combination of the heat treatment's temperature, the thickness of the resist, and the material of the resist can change the size of the resist pattern at the end of the treatment. Especially, as an example of the size change, it often happens that the size of the resist pattern becomes larger than its desired size because of thermal sag. In this case, there is a possibility that the lens properties will vary and furthermore the miniaturization of the lens will be prevented.
In the method for manufacturing a microlens described in Japanese Patent Publication JP-A-05-136460, the nearly hemispheric resist pattern is formed by the potting method. Therefore, it is thought that the size of the resist pattern in this method tends to vary more widely than that in the reflow method. Also, in the method for manufacturing a microlens described in the Japanese Patent Publication JP-A-06-104480, a heretofore known reflow method is used to form a microlens. Therefore, it is thought that the size of the resist pattern tends to vary.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved method for manufacturing a microlens. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.